Voice over internet protocol (“VoIP”) communication systems allow a user of a device to conduct a communication event, such as a call, via a packet-based communication network such as the Internet. To use VoIP, the user installs and execute client software on a user device. The client software provides the VoIP connections as well as other functions such as registration and authentication. In a call conducted using the clients, audio data is transmitted and received between the clients so that users participating in the call can hear each other.
In addition to voice communication, the client may also provide video calling functionality, such that video streams carrying video images of the participants can be transmitted and received between the clients via the network. The participants are able to view the video images of the other participants in addition hearing their voices. This provides a more natural form of communication between the participants, as facial expressions are also communicated, thereby making video calls more comparable to a face-to-face conversation. The video images can be call video images i.e. images of the users themselves captured at their devices, but they can also be other types of video image (that is, other types of video content) in order to implement other communication event modalities, such as screen sharing, document sharing, whiteboard sharing etc. For example, at least one of the video streams may carry a video image of content currently being displayed one of the participant's displays so that it is visible to the other participants.